arXiv subjects

Hamza Boumaza

Publications and source records attributed to Hamza Boumaza.

12 recordsLinked to original sources

arXiv2026

Axial Quasi-normal Modes of Boson-Fermion Stars

We study axial quasi-normal modes of boson-fermion stars composed of ordinary nuclear matter and a self-interacting bosonic dark matter component. The equilibrium configurations are obtained by solving the coupled two-fluid Tolman-Oppenheimer-Volkoff equations, where the neutron sector is modeled with several realistic equations of state and the bosonic sector is described by a repulsively self-interacting complex scalar field in the strong-coupling regime. We analyze linear axial perturbations governed by a Regge-Wheeler type equation whose effective potential reflects the combined matter distribution. Using a continued-fraction method, we compute the complex eigenfrequencies of the fundamental and overtone $w$ modes. We obtain the quasi-normal mode spectrum and investigate its dependence on the dark matter particle mass, self-coupling, and the central densities of both fluids for several realistic neutron star equations of state. We find that increasing the dark matter fraction shifts the oscillation frequencies and damping times. It can also reorder the mode hierarchy through crossings, and it drives a continuous transition from neutron star-like to boson star-like ringdown behavior. Our results demonstrate that the ringdown gravitational-wave signal from post-merger compact objects could encode clear imprints of a dark matter component, offering a new probe of the dark sector with future gravitational-wave observatories.

hep-ph↗

arXiv2026

Neutron stars with primary scalar hair

We investigate static and spherically symmetric neutron star solutions endowed with primary scalar hair in a subfamily of Degenerate-Higher-Order-Scalar-Tensor (DHOST) theories of gravity. By solving the modified Tolman-Oppenheimer-Volkoff (TOV) equations, we construct equilibrium configurations for polytropic and realistic equations of state and analyse the impact of the scalar hair on the stellar structure. We examine the resulting metric and scalar field profiles as well as the mass-radius relation, showing deviations from the predictions of General Relativity (GR). Positive scalar charges lead to more compact stars than in GR and, above a critical threshold, to singularities. Observations could therefore put stringent constraints on the parameters characterising the beyond-GR effects in these theories and their potential scalar hair.

gr-qc↗

arXiv2026

Radial Oscillations of Neutron Stars with Vector-Induced Scalar Hair

In this paper, we investigate the equilibrium configurations and radial perturbations of neutron stars within a subclass of gauge-invariant Scalar-Vector-Tensor (SVT) theories. By solving the generalized Tolman-Oppenheimer-Volkoff (TOV) equations for several values of the modified gravity parameter, we examine the impact of the vector-curvature coupling on the structure and properties of neutron stars. We then extend our analysis by deriving the quadratic action governing linear radial perturbations and computing both the normal modes associated with the matter sector and the scalar quasinormal modes arising from the additional propagating degree of freedom of the theory, which is able to propagate outside the neutron star. Our results show that the modified gravity parameter can significantly affect the mass-radius relation, the oscillation spectrum, and the stability properties of neutron stars, while preserving the coincidence between the onset of radial instability and the maximum-mass configuration, as in General Relativity.

gr-qc↗

arXiv2025

Neutron stars in degenerate higher-order scalar-tensor theories: Axial perturbations

We study the axial (or odd-parity) perturbations of neutron stars in a one-parameter subclass of Degenerate Higher-Order Scalar-tensor (DHOST) theories. After recalling the equilibrium neutron star configurations obtained in a previous work by solving the generalised Tolman-Oppenheimer-Volkoff equations in DHOST theories, we derive the action at quadratic order in linear perturbations of axial type. We then compute the quasi-normal modes (QNMs) for several values of the modified gravity parameter and various equations of state, observing deviations of both frequencies and damping times with respect to general relativity. We also analyze the impact of our modified gravity parameter on the universal relations relating the (rescaled) frequencies and damping times to the compactness of the neutron star.

gr-qc↗

arXiv2025

Scalar quasinormal modes of gravastars

In this work, we present a detailed investigation of gravastars within the framework of scalar-tensor theories, emphasizing both the background and perturbed levels for trivial and non trivial scalar field. We derive and analyze the background equations governing the equilibrium configurations of gravastars by considering the interaction between matter and scalar field through a conformal transformation. By selecting two forms of the coupling function, we identify distinct physical characteristic that leads to deviations from predictions made by general relativity. We show that that the scalar field has a considerable influence on the compactness parameter. By extending our study to the perturbed level, we show also that quasinormal modes are considerably impacted by the presence of scalar field. New and interesting solutions are found, which are absent in general relativity.

gr-qc↗

arXiv2024

Neutron Stars in Scalar Torsion Theories with Nonminimal Coupling

We study the existence and structure of static and slowly rotating neutron stars (NSs) in a particular truncation of scalar torsion theory with a scalar field $ ϕ$ non-minimally coupled to the torsion scalar, and a potential of the form $ V(ϕ)=-μ^2ϕ^2/2 +λϕ^4 /4 $. We derive the hydrostatic equilibrium equations in the static case and solve them numerically for both interior and exterior regions using the appropriate boundary conditions near the center and far from the star. We plot the radial profiles of the metric functions and the scalar field as well as the mass-radius diagram of the star employing a set of four different realistic equations of state (EoS). Our findings show a high degree of compatibility with the observational constraints of the GW170817 event and indicate a maximum mass $2.37M_{\odot}$ obtained with the BSk21 EoS for a coupling parameter $ ξ=0.25 $. We extend our analysis to include slow rotation, and determine the relation between the star's moment of inertia and its mass. We also show that the universality relation of the two forms of the normalized moment of inertia continue to hold in scalar torsion theory with non minimal coupling.

gr-qc↗

arXiv2023

Axial and polar stability of neutron stars in scalar-tensor theories with disformal coupling

In the present work, we study the radial and non-radial perturbative stability of neutron stars in which the matter is disformally coupled to the metric. First, we derive the gravitational and the fluid equations of the neutron star in a static and spherically symmetric background. Then, we calculate the second-order expansion of the action that describes the dynamics of both axial and polar modes. From the resulting expressions, we derive the conditions to avoid gradient and ghost instabilities at the center of the star and at spatial infinity. In addition, a numerical analysis is performed to investigate the stability of a particular model with a constant disformal function denoted as $Λ$ in the whole space time. We found that the chosen model is stable against the gradient instability in a small range of the constant $Λ$.

gr-qc↗

arXiv2022

Neutron stars in degenerate higher-order scalar-tensor theories

We study neutron star configurations in a simple shift-symmetric subfamily of degenerate higher-order scalar-tensor (DHOST) theories, whose deviations from General Relativity (GR) are characterized by a single parameter. We compute the radial profiles of neutron stars in these theories of modified gravity, using several realistic equations of state for the neutron star matter. We find neutron stars with masses and radii significantly larger than their GR counterparts. We then consider slowly-rotating solutions and determine the relation between the dimensionless moment of inertia and the compactness, relation that has the property to be almost insensitive to the equation of state.

gr-qc↗

arXiv2021

Tidal love number of neutron stars with conformal coupling

In this present work, we suggest studying neutron star with a conformal coupling by using the analytical expressions of realistic equations of state. The equations of perturbed and nonperturbed metrics, scalar field and the pressure of the matter are derived in order to observe the effects of the conformal coupling on the mass, radius and tidal love number of polar and axial types. The numerical and the analytical analysis show a different comportment of our model from GR by varying the value of the parameters in the model. We showed also for a particular case that the scalar field becomes vanished outside the star which will give us the opportunity to investigate the behaviour tidal love number of our model. Finally, a comparison between the results of our model and the outcome from the signal of gravitational waves, coming from the fusion of binary neutron star, was performed using the universal relations $C$-$Λ$.

gr-qc↗

arXiv2021

Axial perturbations of neutron stars with shift symmetric conformal coupling

In this present work, the axial quasi-normal modes of neutron stars, with a shift symmetric conformal coupling, are studied for different realistic equations of state. First, we derive the background equations in static and spherically symmetric spacetime and then we solve them numerically by taking into account the continuity and regularity conditions. Second, we extend our calculus to the perturbed level where we derive the equations of motion as well as the ghost and Laplacian instability. We find that the proposed model is free from these instabilities everywhere. We find also that the variation of the quasi-normal modes is affected by the conformal coupling where the deviation from general relativity is observed. Finally, with this motivation, we fit our numerical result with universal relations for axial quasi-normal modes using five type of realistic equations of state. The universality of the scaled frequency and damping time in terms of the compactness in this model is confirmed in this model.

gr-qc↗

arXiv2020

Late time cosmological evolution in DHOST models

We study the late cosmological evolution, from the nonrelativistic matter dominated era to the dark energy era, in modified gravity models described by Degenerate Higher-Order Scalar-Tensor (DHOST) theories. They represent the most general scalar-tensor theories propagating a single scalar degree of freedom and include Horndeski and Beyond Horndeski theories. We provide the homogeneous evolution equations for any quadratic DHOST theory, without restricting ourselves to theories where the speed of gravitational waves coincides with that of light since the present constraints apply to wavelengths much smaller than cosmological scales. To illustrate the potential richness of the cosmological background evolution in these theories, we consider a simple family of shift-symmetric models, characterized by three parameters and compute the evolution of dark energy and of its equation of state. We also identify the regions in parameter space where the models are perturbatively stable.

astro-ph.CO↗

arXiv2019

Growth of Matter Perturbations in the Bi-Galileons Field Model

We study a dark energy cubic bi-Galileons field model based on truncation of the recently proposed generalized covariant multi-Galileons model. We investigate the cosmological dynamic of the model by the theory of dynamical systems through the analysis of the properties of the fixed points in each cosmological epoch. We show the existence of two tracker solutions, one of which is that of the cubic single Galileon model and the other solution is the signature of the second Galileon field. Exploiting the competition between the two Galileon fields, we find a dark energy solution that avoids the approach to the tracker solution with dark energy equation of state $w_{DE}=-2$ during the matter epoch which is disfavored by the observational data. We study also the growth rate of matter perturbations. Using recent $fσ_{8}$ redshift space distortion (RSD) and model-independent observational Hubble (OHD) data sets, we place observational constraints on the coupling constant and cosmological parameters of the bi-Galileons model through Monte Carlo numerical method based on the Metropolis-Hastings algorithm. We find that the amplitude of growth matter fluctuations is consistent with the Planck15 data and ease the tension between early and later clustering, and fits better the data from the DES survey over the data from KiDS-450 survey. We also find that the best fit value for the Hubble constant is compatible with new measurements of Cepheid-supernovae distance scale. Finally, we perform a model selection through the Bayes factor and found that the bi-Galileons model is disfavored in comparison to the $Λ$CDM model, but slightly preferred to $w$CDM model.

astro-ph.CO↗